Gas analyzers are currently found on the market which basically operate in accordance with four different principles:
(a) Mass spectrometry; PA1 (b) Raman spectrometry; PA1 (c) IR-spectrometry; and PA1 (d) Change of mass of quartz crystals.
Mass spectrometers are rather expensive, but they are capable of rapidly measuring all of the gases, and of doing so with relatively good accuracy. They do, however, require qualified operators.
Raman spectrometers are also quite expensive, and their use in this field is relatively recent. Thus their quickness, accuracy and stability have not yet been documented. Furthermore, they also presumably require qualified operators.
The most commonly used such devices are the IR-based instruments. They are relatively cheap and stable, and do not require qualified operators. Their accuracy, however, depends on their design. For carbon dioxide and laughing gas (N.sub.2 O), the instruments presently on the market are sufficiently rapid for end-tidal measurements, but up to now these instruments have been too slow for anesthesia gases.
Instruments based on quartz crystals are very cheap. They exist, however, only for anesthesia gases. While they are rapid, they are unfortunately unstable, and have a rather short life. They also have limited accuracy, presumably because of the influence of laughing gas and water vapor thereon.
Measuring systems utilizing infrared detection are based upon the principle that the substance which is to be analyzed absorbs IR-radiation of wavelengths which are unique for the particular substance in question. In these systems, IR-radiation is generated by a source of radiation which continuously emits radiation over a large wavelength range. By means of an optical filter, for example, the correct wavelength is selected, and the radiation is then detected by an IR-detector which transduces the incident radiation energy to a proportional electrical signal. The choice of the analytical wavelength for the particular substance in question is very critical and wholly decisive in determining system characteristics such as accuracy and speed. In practice, this choice is governed by whether several of the substances present in a given sample absorb at or near the chosen wavelength, and whether it is technically and/or economically possible to measure radiation of this wavelength to the degree of accuracy required therefor.
Existing IR instruments for carbon dioxide and laughing gas (N.sub.2 O) have now been optimized to such a degree that improvements which can be translated into better performance for these substances do not appear to be possible.
For anesthesia gases, the situation is quite different. The absence of suitable commercial IR-detectors in this field has made it necessary for IR instruments for such gases to operate with short wavelengths, where these gases have very low absorption. This, in turn, has resulted in these instruments' requiring large sample volumes, and these instruments are therefore quite slow. Moreover, measurements made by these instruments have been disturbed by the presence of water vapor in the gases so analyzed.
For several years now, however, detectors have been on the market which allow for measurement at longer wavelengths, where the gases absorb much more strongly. Instruments which make use of these longer wavelengths therefore require very small sample volumes and are consequently far more rapid.
For end-tidal measurement the measuring system must somehow be provided with means to receive information indicating the approach of the end of the expiratory phase. This information can be obtained by measurement of the carbon dioxide content of the expired gas, and instruments which can thus measure the end-tidal carbon dioxide content have been known for a long time (e.g., capnometers, capnographs, etc.). On passage through the carbon dioxide meter, however, the sample gas becomes disturbed and an anesthesia gas meter coupled in series therewith will thus receive a phase-displaced and distorted sample. That is to say, any curves recorded thereby will have a distorted profile.